Umbrella companies, like all employers in the UK, are responsible for paying employer NICs.
Umbrella companies are paid a fee by an employment agency to supply a worker to its end client. From that fee they pay their own liabilities such as employer NICs. The remainder after those payments forms the worker’s gross pay, from which the worker pays Income Tax and employee NICs. Like other employers, an umbrella company is required to account for the Income Tax and employee NICs directly to HMRC under Pay As You Earn.
In April 2020 the Government legislated to introduce a key facts page for agency workers to clarify how their terms of engagement affect their pay prospects. The key facts page must explain the deductions from pay.
Further information is on GOV.UK: www.gov.uk/government/publications/providing-a-key-information-document-for-agency-workers-guidance-for-employment-businesses.